The invention relates to acoustical imaging interferometers, and more particularly to passive/active hybrid methods and systems for acoustical imaging of buried underwater objects.
Detection of buried objects, such as mines by any means is difficult. In a land environment, mines remaining from a war are a scourge in many countries of the world. In the underwater environment, sea mines can be equally devastating, denying coastal waters to fisherman and civilians.
In time of war, military operations are also impeded or denied by the presence of sea mines. Mines that are tethered below the water surface or are standing proud on the seafloor may often be detected by existing means or by systems in advanced development. Acoustical imaging methods are effective for tethered and proud mines even in very turbid littoral waters, where optical methods often fail. Low frequency sonar systems may also be employed and do not have the limitations of optical systems in turbid water. FIG. 1 provides a diagrammatic presentation of the relative turbidity versus resolution potential of conventional sonar, acoustical and optical imaging systems.
Many sea mines are designed to progressively bury themselves through the scouring action of water movement on the seafloor. Optical, sonar and conventional acoustical imaging systems are not effective for these mines. In addition, many sea mines are constructed of non-magnetic or non-metallic materials such as fiberglass composites and are virtually undetectable by electromagnetic means.
Partially or completely buried mines remain an exceptionally difficult detection problem, for which there has been no solution prior to this invention. Low frequency excitation of the surface of the ground together with surface displacement measurement is an approach that has produced promising laboratory results for land mines in an on-land environment.
It is a goal of the invention to provide improved methods and apparatus for detecting partially and fully buried objects on the seafloor, in particular man-made objects. It is another goal to use imaging techniques requiring no light. It is a further goal to employ three dimensional techniques that facilitate discriminating loose seafloor materials from larger, more solid objects partially or fully submerged or buried immediately beneath the seafloor, and determining the size and shape of an object.
It is also a goal to employ active techniques such as acoustical pulses to disturb the elastic seafloor materials to reveal by their relative non-motion, the presence of objects buried immediately below the seafloor. It is a yet further goal to use three dimensional acoustical imaging techniques to capture before and after images interspersed with at least one acoustical pulse applied so as to disturb and cause some detectable degree of movement of the elastic seafloor materials that will be evident in the after image.
To this end there is assembled on an underwater platform an acoustic transducer array camera with range-gated response that enables three dimensional volumetric images to be constructed from a series of closely spaced planar response images associated with an original transmitted echo pulse. The platform is positioned in proximity to the seafloor target area of interest. The camera is operated in coordination with another acoustical transducer which can apply a substantial, low frequency tone burst or single acoustical pulse to the seafloor so as to cause elastic seafloor material to move a detectable amount, but has little or no effect on objects of relatively higher elasticity or density compared to the seafloor material. Images taken before and after applying the seafloor pulse are compared for evidence of movement and non-movement, yielding a profile of the buried object. Recognizable shapes and sizes help the operator or a computer to discriminate between man-made and natural objects.
Other goals and objectives will be readily apparent to those skilled in the art from the description and figures that follow.